Plants are known (EP-A-0,795,616) which are intended for the continuous heat treatment of metal strip, in which the strip to be treated travels through an oven consisting of a plurality of heat-treatment sections, especially heating sections, temperature-hold sections, annealing sections, cooling sections, etc. In this publication, the present proprietor has recommended the use of heating and/or cooling enclosures or chambers in which there is an atmosphere consisting of a hydrogen-based gas mixture, the hydrogen content of which is greater than the values usually permitted so as to increase the cooling rate. Thus, according to this prior technique, a gas mixture, especially a nitrogen/hydrogen gas mixture, having a hydrogen content which is greater than 15% and which may reach 50%, i.e. greater than the explosibility limit value for this gas, may be used as the atmosphere in the cooling enclosure.
In the cooling enclosure of the abovementioned plants, the recycled atmosphere gas is blown onto the continuously moving metal strip, after it has been cooled especially with the aid of gas/water heat exchangers, the composition of the atmosphere gas, and more particularly its high hydrogen content, making it possible to obtain very rapid cooling rates of the order of 100.degree. C. per second. The plants therefore comprise a circuit of ducts for recirculating the N.sub.2, H.sub.2 atmosphere gas, one or more fans for continuously circulating this atmosphere gas in the recirculation ducts, and gas/water exchangers.
Given the very high hydrogen content of the atmosphere gas used during the rapid cooling, it is necessary to take great precautions to ensure that the plant is safe; indeed, a leak, however tiny, of the atmosphere gas into the environment cannot be tolerated.
The present invention aims to provide a solution to this safety problem, especially by taking into account the three following main phenomena which can impair this safety:
1) it may happen that the strip breaks inside the oven and, more particularly, in the rapid-cooling chamber. In this case, the atmosphere gas contracts, this being caused directly by the interruption in the heat supplied by the strip and the continuity of the gas/water exchange through the exchanger or exchangers. This atmosphere-gas contraction phenomenon results in a sudden depressurization in the rapid-cooling chamber and in the oven; PA1 2) the assembly of ducts for circulating the atmosphere gas includes compensators which allow the ducts to expand. In the event of a break in a corrugation of a compensator, it is absolutely necessary to prevent air from getting into the atmosphere-gas recirculation circuit or any leakage of this gas into the environment; and PA1 3) during a break in the power supply to the plant or during a general failure of the supply mains for the factory where this plant is located, it is necessary to stop recirculation of the atmosphere gas immediately. PA1 a main flow area via which the cooling gas flows through the said exchangers, this flow area being provided with a register upstream of the said exchanger or exchangers; PA1 bypass flow areas allowing the flow of cooling gas to be diverted in the event of the main flow area being closed, so that this flow does not pass through the said exchanger or exchangers, the said bypass flow areas being provided with registers; and PA1 actuators with a very short response time, i.e. less than five seconds, these being tripped by means for detecting a break in the strip, a break in a compensator corrugation and a break in the power supply.
The present invention solves the abovementioned problems by providing means making it possible, on the one hand, to prevent the atmosphere gas from passing through the exchanger or exchangers, i.e. preventing too rapid a cooling of this atmosphere gas, thereby avoiding a sudden depressurization in the oven should the strip break and, on the other hand, to stop, in a very short time, i.e. less than five seconds, the recirculation of the atmosphere gas in the rapid-cooling chamber, during a break in the corrugations of a compensator or during a break in the power supply.
Consequently, the present invention relates to a rapid-cooling oven, which may form part of a plant comprising various treatment stations, especially stations for the heat treatment of products in strip form, through which a metal strip runs continuously, the rapid cooling being obtained by spraying onto the strip a jet of atmosphere gas consisting of a nitrogen/hydrogen mixture having a hydrogen content which is greater than 15%, and preferably close to 50%, the atmosphere gas being recirculated using a circuit of recirculation ducts comprising at least one fan and at least one gas/water exchanger, the various ducts comprising compensators made in the form of a series of corrugations allowing all the ducts to expand, this oven being characterized in that the enclosure, in which the said exchanger or exchangers are housed and into which the atmosphere-gas recirculation ducts emerge, comprises:
According to one embodiment of the invention, the actuators, which are preferably pneumatic, are tripped by a sensor which detects any variation in the tension in the strip.
According to another embodiment of this invention, the actuators are tripped by a sensor which detects any variation in the pressure in the rapid-cooling chamber.
According to the present invention, means designed and produced so as to allow the rotation of the fan or fans to be stopped in a very short time are also provided, it being possible to produce these means by coupling the fan to a generator to which it is coupled either in the event of a break in a corrugation of a compensator or in the event of a break in the power supply.
Other features and advantages of the present invention emerge from the description given below with reference to the appended drawings which illustrate an embodiment thereof, given solely by way of example.